The present invention concerns methods and apparatus for stretching planar workpieces, and more particularly an improved system for stretching a printing screen.
In many instances, it is important to be able to quickly stretch a sheet of planar material to a desired tension or desired dimensions. For example, screen printing requires that a mesh screen of metal or fabric be uniformly stretched under very high tension while ink is applied through the mesh of the screen. Uniform tension on all parts of the screen is critical to ensure uniform distribution of ink through the screen and to produce a high quality image. To achieve uniform tension on the screen, prior devices have applied clamps to the edges of a screen at discrete spaced locations. The clamps are then pulled outwardly to exert tension on the screen in all directions until the screen is taut.
Very high tensile forces must be applied through the clamps to the screen to achieve tension of a sufficient magnitude in such devices. Rapid stretching of the screen by the clamps can, however, exceed the tensile strength of a screen and damage the integrity of the mesh. Hence, prior screen printing processes have required that the screen be stretched very slowly, often over many hours. This delay in screen stretching impairs printing efficiency and increases the cost of screen printing processes.
Several frames are known for holding mesh screens used in screen printing processes. U.S. Pat. No. 2,903,967; U.S. Pat. No. 3,863,368; U.S. Pat. No. 3,477,574; U.S. Pat. No. 3,762,080; U.S. Pat. No. 4,452,138; and U.S. Pat. No. 4,702,783 disclose planar frames that exert tension in the plane of the frame to stretch a screen taut. U.S. Pat. No. 4,144,660 and U.S. Pat. No. 4,190,974 also disclose planar frames for stretching a workpiece, such as an artist's canvas. U.S. Pat. No. 2,752,630 describes a star-shaped, planar tensilizer for stretching thermoplastic films. Finally, U.S. Pat. No. 2,042,874; U.S. Pat. No. 2,455,640; and U.S. Pat. No. 2,604,725 disclose planar frames for stretching curtains.
Such devices have many uses. But, all such devices have drawbacks which limit their usefulness for precision work, such as stretching mesh screens for use in printing. In particular, many of the prior devices require a highly trained worker to complete the installation over a long period of time. With prior screen printing equipment, for example, it typically takes hours to gradually tension a screen. And, even skilled workers are prone to damage printing screens. Other of the devices are too imprecise to be useful.
Accordingly, it is an object of this invention to provide an improved frame for quickly and efficiently stretching a planar workpiece, such as a mesh screen.
It is another object of the invention to provide such an improved frame that uniformly applies tension to the workpiece.
Also, it is an object of the invention to provide a stretching system that requires only minimal training to use.
A further object is to provide modular workpieces, particularly printing screens, that can be repeatedly installed and removed from a frame.
More specifically, it is an object to provide workpieces that can be repeatedly installed at the same position and tension.
These and other objects of the invention will be understood more clearly by reference to the following detailed description and drawings.